1. Field of the Disclosure
This disclosure relates to a method of geophysical prospecting which improves the accuracy of depth velocity model building and seismic migration. Specifically, the disclosure uses a zero-offset and walkaway VSP, multi-azimuth or 3D VSP survey for determination of anisotropy parameters characterizing subsurface velocities that may be used for imaging of reflections.
2. Description of the Related Art
In surface seismic exploration, energy imparted into the earth by a seismic source reflects from subsurface geophysical features and is recorded by a multiplicity of receivers. This process is repeated numerous times, using source and receiver configurations which may either form a line (2-D acquisition) or cover an area (3-D acquisition). The data which results is processed to produce an image of the reflector using a procedure known as migration.
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long and complicated travel paths between source, reflector, and receiver.
Prior art methods have used a walkaway Vertical Seismic Profile (VSP) survey to estimate formation velocities within and below the receiver array. Common to the prior art methods is an assumption that the earth is isotropic. It has been recognized for several years that the earth is anisotropic, i.e., that the velocity of compressional waves depends upon the direction of propagation.
P-wave anisotropy is manifested by a change in the compressional wave velocity with direction of propagation in earth formations due to combined effects of sedimentary layering and the intrinsic anisotropy of the rock. Shales, in particular, could exhibit more than a 20% difference in P-wave velocities parallel to bedding and P-wave velocities perpendicular to bedding. Sandstones and limestones usually show smaller differences in velocity with direction of propagation. Postma (1955) showed that a type of anisotropy called transverse isotropy could be exhibited by seismic waves propagating through a thin layering of isotropic materials.
Determination of anisotropic velocities from surface seismic data is difficult due to the relatively poor data quality and the relatively low frequencies of surface seismic data. Nevertheless, there is prior art on the determination of an anisotropic velocity model for depth imaging of seismic data. See, for example, U.S. Pat. No. 6,864,890 to Meek et al.
The present disclosure addresses the problem of determining anisotropic formation velocities using a walkaway VSP survey, multi-azimuth walkaways or 3D VSP survey. In a walkaway VSP survey, measurements are made using a plurality of receivers in a borehole responsive to excitation of at least one seismic source at a plurality of distances from the wellbore. The estimated velocities may then be used for migration of the walkaway(s)/3D VSP data or of surface seismic data and depth velocity model building. This method is particularly useful in the drilling of offset wells where an initial well that may or may not be productive has been drilled. Using the method of the present disclosure, it is possible to image the subsurface of the earth away from the initial well.